package com.mongodb.util;

import java.util.concurrent.atomic.AtomicLong;

/**
 * 雪花算法
 */
public class SnowflakeIdUtil {
    // 起始的时间戳
    private final static long START_TIMESTAMP = 1672502400000L; // 2023-01-01 00:00:00

    // 每一部分占用的位数
    private final static long SEQUENCE_BIT = 12; // 序列号占用的位数
    private final static long MACHINE_BIT = 5;   // 机器标识占用的位数
    private final static long DATACENTER_BIT = 5; // 数据中心占用的位数

    // 每一部分的最大值
    private final static long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);
    private final static long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private final static long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);

    // 每一部分向左的位移
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTAMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

    private static long datacenterId = 0;  // 数据中心
    private static long machineId = 0;     // 机器标识
    private static long sequence = 0L; // 序列号
    private static final AtomicLong lastTimestamp = new AtomicLong(-1L);

    public SnowflakeIdUtil(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }

    /**
     * 生成下一个ID
     * @return 下一个ID
     */
    public static synchronized long nextId() {
        long currentTimestamp = currentTimeMillis();

        if (currentTimestamp < lastTimestamp.get()) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id for " + (lastTimestamp.get() - currentTimestamp) + " milliseconds");
        }

        if (currentTimestamp == lastTimestamp.get()) {
            // 相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            if (sequence == 0L) {
                // 同一毫秒的序列号溢出，等待下一毫秒
                currentTimestamp = tilNextMillis(lastTimestamp.get());
            }
        } else {
            // 不同毫秒内，序列号置为0
            sequence = 0L;
        }

        lastTimestamp.set(currentTimestamp);

        // 时间戳部分 | 数据中心部分 | 机器标识部分 | 序列号部分
        return ((currentTimestamp - START_TIMESTAMP) << TIMESTAMP_LEFT)
                | (datacenterId << DATACENTER_LEFT)
                | (machineId << MACHINE_LEFT)
                | sequence;
    }

    /**
     * 阻塞到直到下一毫秒，以获得新的时间戳
     * @param lastTimestamp 上次生成ID的时间戳
     * @return 当前时间戳
     */
    private static long tilNextMillis(long lastTimestamp) {
        long timestamp = currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = currentTimeMillis();
        }
        return timestamp;
    }

    /**
     * 获取当前时间戳
     * @return 当前时间戳
     */
    private static long currentTimeMillis() {
        return System.currentTimeMillis();
    }

    public static void main(String[] args) {
        SnowflakeIdUtil idGenerator = new SnowflakeIdUtil(1, 1);
        for (int i = 0; i < 10; i++) {
            System.out.println(idGenerator.nextId());
        }
    }
}
